Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
G. 336, 337.] 
IV. Mechanism of Accommodation 
In the investigation of the extracapsular mechanism of accommoda¬ 
tion it is well first to review the known facts that have been ascertained 
for the human eye. 
Next to the change of curvature of the surfaces of the lens and the 
increase of thickness of the lens, Helmholtz attached most importance 
to the contraction of the pupil accompanying accommodation. It has 
been a question of debate whether this phenomenon is associated with 
accommodation or with convergence of the lines of vision. Concerning 
this, clinical observations on eyes in which accommodation was com¬ 
pletely paralysed as a result of diphtheria have shown, that contraction 
of the pupil occurs in convergence, and that accommodative contrac¬ 
tion of the pupil may be unhampered when convergence is absent. 
It seems most probable that the three innervations are associated with 
one another and are released by the impulse for clear vision of near 
objects. (Simply because the pupil becomes contracted by mechani¬ 
cal means when the anterior chamber is drained is no reason for in¬ 
ferring, as Tscherning does, that the contraction of the pupil in ac¬ 
commodation is produced by mechanical means.) The function of the 
contraction of the pupil during accommodation is to be seen in the 
increase of the depth of focus, since the changes of accommodation 
requisite for seeing near objects are essentially diminished by the in¬ 
creased depth. This has nothing to do with aberration, as will be shown 
further on. What advantages this contraction of the pupil offers, might 
have been better understood before the invention of spectacles, be¬ 
cause in those days it afforded the only recourse against normal 
The best way at present to observe the changes in the form of the 
anterior chamber during accommodation is with the binocular magnify¬ 
ing glass invented by Czapski. The experiments described by Helm¬ 
holtz do not always give positive results, for reasons presently to 
be discussed. In the stereoscopic image obtained with the instru¬ 
ment above mentioned, it is easy to establish definitely not only the 
protrusion of the central parts, but also (at least in the case of young 
people with sufficient accommodation) the recession of the periphery. 
The latter effect is a necessary consequence of the former, since the 
volume of fluid contained within the anterior chamber cannot be re¬ 
duced by flowing over into the posterior chamber on account of the 
physiological adherence of the iris to the anterior surface of the lens 
capsule. But the periphery of the iris does not move as*much as the 
centre, because the fluid that escapes from the central region takes 
up in the periphery a space of larger diameter and consequently of 
less thickness. This does not indicate that the mechanism of the peri-


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